Air borne particulate, either occurring naturally, such as pollen and dust, or generated through industry, such as smoke or automobile exhaust, can be hazardous to human health. Particulate concentration and content may typically be monitored to, among other things, evaluate the changes in particulate matter due to natural or industrial activity. Particulate sampling systems are typically used to collect and categorize the particulate content of ambient air. One typical particulate sampling system is provided by the Environmental Instruments Division of Thermo Electron Corporation of East Greenbush, N.Y. and described in U.S. Pat. No. 6,502,450, the disclosure of which is incorporated by reference herein in its entirety. The particulate sampled by such systems is typically characterized by the size of the particles collected, for example, particulate matter (PM) having diameters less than or equal to 10 microns (μm), that is, particles designated “PM10” or “PM10” particles in the art, or lower may be collected.
When sampling gas using a particulate sampling system it is often desirable to remove as much water vapor as possible from a sampled gas stream before measuring the particulate content, for example, measuring the mass of the particulate. The presence of water vapor in a sampled gas stream can interfere with the accurate measurement of, for example, the mass of particulate matter in the gas stream. The effect of the presence of water vapor upon the measuring device can be particularly acute when the measuring device is operated at temperatures at which water vapor tends to condense on the sensing hardware, for example, at temperatures of about 30 degrees C. or lower. Aspects of the present invention are adapted to remove water vapor from sample gas streams whereby the particulate measuring device can provide a more accurate indication of the particulate content of the sampled gas stream.
In addition, while removing water vapor from the gas stream, it is important that the water vapor removal device or method minimize or avoid undesirable impact upon the particulate matter in the sample stream. For example, conventional water vapor removal devices, that is, “dehumidifiers” or “dryers” designed to remove water vapor from a gas stream, for example, those disclosed in U.S. Pat. Nos. 6,651,480; 6,151,953; 6,171,374; 5,932,795; 5,571,945; and 3,735,559, may typically interfere with the goal of the particulate sampling system, that is, collecting particles. Among other things, conventional dehumidifiers, for example, bundled-tube dehumidifiers, typically provide at least some obstruction to the flow of gas whereby particles are either hindered or captured in the dehumidifier.
Thus, a need exists for dehumidifying devices and methods that effectively remove water vapor from a sample gas stream while minimizing the impact of the devices and methods on the flow of particles in the gas stream. Aspects of the present invention provide some means of effecting the desired dehumidification while minimizing the impact upon the particle content of the gas stream.